Radiant heaters having multiple heating zones are known for example from United Kingdom Patent Specification No. 2 069 300 and European Patent Specification No. 0 103 741. European Patent Specification No. 0 103 741 describes a heater having inner and outer concentric heating zones, the inner heating zone containing one heating element and the outer heating zone containing two heating elements. A temperature sensor of a thermal cut-out device extends over both the inner and outer heating zones and is sensitive to heat emitted in both zones. The thermal cut-out device has two switches operating at upper and lower cut-out temperatures in order to protect the glass ceramic cooking surface against overheating.
When the inner heating element is used alone, for example to heat a small cooking utensil, the inner heating element is operated at full power. In this condition, the inner heating element is connected to the thermal cut-out device by way of its switch operable at the lower cut-out temperature. When both the inner and outer heating zones are to be used together, for example to heat a large cooking utensil, one of the heating elements in the outer zone is electrically connected in series with the heating element in the inner zone, and the two heating elements in series are connected in parallel with the other heating element in the outer zone. In this condition, the heating elements are connected to the thermal cut-out device by way of its switch operable at the upper cut-out temperature. The effect of this is to reduce the specific heating surface loading in the inner zone as compared with the outer zone.
This arrangement has the disadvantage that two switches on the thermal cut-out device are required to control the operation of the heating elements, one of the switches being a changeover switch rather than a simple make-and-break switch. This precludes the possibility of using the second switch on the thermal cut-out device as a signal switch, for example to warn the user of the cooking appliance that the glass ceramic cooking surface is at an elevated temperature and may be too hot to touch.